The manufacture of self-bonded polycrystalline sintered compacts from pulverulent europium hexaboride with a stoichiometric composition which has adequate stability and density for use in nuclear technology is difficult.
Pressureless sintering tests produced discouraging results. In those tests, starting powders having particle sizes of less than 5 .mu.m were predensified without a binder, to form green bodies having from 50 to 60% of the theoretical density (hereinafter % TD) and then heated under vacuum. After 16 hours at sintering temperatures of from 2230.degree. to 2730.degree. C., sintered articles having densities of only 63% TD were obtained. A weight loss of about 40% occurred in the process. At lower sintering temperatures (less than 2230.degree. C.), no densification or shrinkage was observed.
By hot pressing at 2230.degree. C. in vacuo, sintering densities of 90% TD could be obtained although considerable grain coarsening was observed, as well as a reaction between the sintered compact and the graphite mould (cf. E. W. Hoyt et al in General Electric Report GEAP-3332, Contract No. AT (04-3)-189, Atomic Energy Commission, June 6, 1960).
Hot pressing at temperatures in the range of from 1850.degree. to 1950.degree. C. produced sintered bodies with densities up to 99% TD. However, a reaction between the sintered compacts and the graphite mould was reported. The resulting sintered compacts were single-phase that is, no secondary phases could be detected. However, they had bimodal grain distribution in the microstructure which comprised a matrix having grain sizes of less than 10 .mu.m surrounded by larger grains of up to 200 .mu.m (cf. A. E. Pasto et al in Trans. Amer. Nucl. Soc., vol. 26 (1977), page 176). In spite of their high density, shaped articles having such a non-homogeneous microstructure are not especially stable.
Further reduction of the sintering temperature to 1700.degree. C. during hot pressing reduced the reaction with the graphite mould and permitted better control of the grain size. However, the lower sintering temperature made the use of submicron EuB.sub.6 powder necessary. The submicron EuB.sub.6 powder is manufactured by grinding in a tungsten ball mill for 76 hours which is not only uneconomical but introduces contamination into the sintered compact by material abraded during grinding.
It has also been established that the problems encounted during hot pressing are connected with the presence of free europium. This can be rectified by increasing the boron content for example, by adding boron or boron carbide to provide an excess boron phase. Pellets which had been manufactured from a mixture of EuB.sub.6 and 5% boron sintered at 1700.degree. C. had a porosity of 8% and had a finer microstructure than did those which had been sintered at 2150.degree. C. under otherwise identical conditions (cf. G. W. Hollenberg et al in Ceramic Bulletin, vol 60 (1981), pages 478-480 and report in Chem. Abs., vol 92 (1980), No. 46194 u).
It was only possible to achieve adequate densification of EuB.sub.6 powders of stoichiometric composition without adding binders by means of hot pressing. Since hot pressing makes economic sense only if it can be carried out using customary graphite moulds, the possible reactions of EuB.sub.6 with C or B.sub.4 C, and the phase compositions in the ternary system of Eu--B--C, were investigated in detail, In the section EuB.sub.6 --C, a temperature-dependent, limited solubility of C in the EuB.sub.6 lattice was found, with the formation of a crystalline phase corresponding to the formula EuB.sub.6-x C.sub.x', in which x has values of from 0 to 0.25. Carbon in amounts of more than 3 percent by weight is not soluble in the EuB.sub.6 lattice and leads to the formation of additional phases which are sensitive to atmospheric moisture. In the section EuB.sub.6 --B.sub.4 C, the above-mentioned crystalline phase was also detected, but no additional phases were found. It is therefore assumed that the section EuB.sub.6 --B.sub.4 C is a pseudobinary system in which these two phases are in equilibrium.
It was concluded from these investigations that when EuB.sub.6 powder of stoichiometric composition is subjected to hot pressing in graphite moulds, carburization of the EuB.sub.6 takes place with the formation of a solid solution of EuB.sub.6-x C.sub.x, in which x has the above-mentioned meaning. If, however, at the sintering temperature selected, further carburization beyond the solubility limit of the carbon takes place, the resulting sintered compacts are not resistant to atmospheric moisture due to the presence of europium carbide or boron carbide phases which are sensitive to hydrolysis. However, by maintaining certain conditions, such as avoiding excessively high sintering temperatures during hot pressing and using fine EuB.sub.6 powders as the starting material, stable, single-phase EuB.sub.6 sintered compacts containing between 0.4 and 0.6 percent by weight of carbon can be obtained (cf. K. A. Schwetz et al in Ceramurgia International, vol. 5 (1979), pages 105-109).
In all the known processes, the target has always been the manufacture of self-bonded polycrystalline sintered compacts from EuB.sub.6 powder of stoichiometric composition without the addition of binders or sintering aids. The prevailing opinion was that self bonding compacts provided the best properties in the end product. The process, however, had known difficulties such as a high weight-loss, in the case of pressureless sintering, as a result of the formation of volatile europium or europium compounds or excessive carburization caused by reaction with the graphite molds in the case of hot pressing. The reaction with the graphite molds not only necessitates further processing of the resulting sintered compacts but is associated with a large amount of abrasion of the mold so that it cannot be used again. These disadvantages encountered in the case of hot pressing can be kept within limits only by means of extraordinary measures such as accurate temperature control and the use of submicron powders.
The economical production of polycrystalline EuB.sub.6 sintered compacts is, however, of great significance for the use of europium hexaboride as a neutron-absorbing material in nuclear technology.
The problem is therefore to provide polycrystalline sintered compacts based on europium hexaboride having properties which are at least as good as those of the known products and which can be obtained without the self-bonding of the EuB.sub.6. In addition, the compacts should have the advantage that they can be manufactured not only by means of hot pressing in graphite molds but by means of known sintering processes such as isostatic hot pressing and pressureless sintering, in a simple and economical manner.